Advanced
Power Supply
Topics
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Welcome to the Advanced Power Supply Topics Web seminar.
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Slide 1
Session Agenda
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Inrush Limiting and Soft-start
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Input Transient Protection
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Under / Over Voltage Lockout
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Overvoltage Crowbar
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Overcurrent Issues
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Voltage Trimming
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Remote Sensing
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Power Supply Sequencing
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Load Sharing Issues
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EMI Issues
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 2
This is the agenda for this course. We will discuss issues that affect the
reliability and saleability of switch mode power supplies.
Inrush current and soft-start functionality affect the turn-on stresses of a
power supply.
Input Transients refer to externally generated excessive voltage pulses that
can damage a switch mode power supply.
Low or high input supply voltages are problems handled with Under and
Over Voltage Lockout functions.
Power supply output Over-voltage and Over-current conditions need to be
considered.
Voltage Trimming, Remote Sense, and Power Supply Sequencing are topics
that affect the output voltage at the user’s load.
In large fault tolerant systems, Load Sharing among power supply modules
is required.
To sell power supplies, EMI (Electro Magnetic Interference) issues must be
considered.
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Inrush Limiting
Controlled charging of input capacitors
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Reduces component stress
Implemented with:
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NTC (Negative Temperature Coefficient Resistors)
TRIACs
Relays
Active PFC circuits
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 3
When a power supply is first plugged in or turned on, there is a large current
surge into the power supply as the input capacitors are charged.
Current inrush causes the “arcing” noise often heard when a computer or
other device is first plugged into the wall socket. Current inrush is stressful
on switches, diodes, capacitors, wall sockets, etc.
Many power supplies incorporate “Inrush Limiting”. The most common
method is the NTC (Negative Temperature Coefficient) resistor. The NTC
resistor has a high resistance (about 40 ohms) when cold and a low
resistance (< 1 ohm) when hot. The NTC resistor is placed in series with the
input to the power supply. The cold resistance limits the input current as the
input capacitors charge up. The input current heats up the NTC and the
resistance drops during normal operation.
If the power supply is quickly turned off and back on, the NTC resistor will be
hot so its low resistance state will not prevent an inrush current event.
Another method is to use TRIACs or relays in parallel with a resistor. The
resistor limits inrush current. A sensor circuit monitors the input capacitor
voltage. When the input capacitors are charged, either a relay or a TRIAC
across the input resistor is enabled to provide a path for power flow during
normal operation.
Active Power Factor Correction (PFC) circuits also minimize inrush currents
by minimizing the size of the input capacitors directly connected to the power
input terminals.
Page 3
Soft-start
Controlled output voltage ramp-up
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Reduces component stress
Reduces acoustic noise
Reduces inrush current
Reduces output voltage overshoot
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 4
Soft-Start is the process of actively controlling the power supply so the
output voltage(s) rise at a controlled rate at power-on. The soft-start process
limits peak currents that normally flow to charge capacitors in the power
supply and the attached load at power-on.
The controlled soft-start p-rocess reduces stresses on components and
helps to reduce input current inrush.
A Hard-Start (non soft start) power-up process will generate severe current
loads that cause acoustical noise from components such as transformers
and capacitors that are audible to the customer.
Hard-start power supplies can experience output voltage over-shoot on startup.
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Input Transient Protection
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Lightning or Equipment generated Surges
1-20 KV amplitude, 0.5 – 50 usec duration
Metal Oxide Varistors (MOV)
Transient Suppressor Diodes (Transzorb)
Gas Filled Surge Suppressor
Gas
Filled
MOV
Transzorbs
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 5
Lightning and electrical equipment on the AC power lines can generate very
high voltage spikes (1 to 20 thousand volts) lasting 0.5 to 50 microseconds.
There are several types of devices used to limit the input voltage transients.
1. The most common transient protection device is the MOV (Metal Oxide
Semiconductor), also called a Varistor. MOVs clamp high voltages by
becoming a low resistance when a high voltage is applied. MOVs are
inexpensive and can absorb a lot of energy. The relatively high “on”
resistance means that “clamped” voltage can still exceed a thousand volts.
MOVs have a wear-out mechanism with repeated energy absorption events.
They fail shorted and therefore they must be fused.
2. Transzorbs are basically power ZENER diodes. Transzorbs offer very
high speed and tight voltage clamping characteristics. Transzorbs have
limited energy absorption and are expensive.
3. Gas filled surge suppressors can clamp very high energy surges, but the
clamping voltage may be several times their rated voltage . Once a gas
filled suppressor arcs and conducts (begins clamping), they will continue to
conduct until the input voltage is removed. When used on an AC power line,
the alternating voltages usually commutate the arc. Gas filled devices are
not used on DC power lines.
Many systems use combinations of these devices to obtain the desired
surge protection.
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Under / Over
Voltage Lockout
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At low input voltages, current grows
causing power dissipation problems.
At high input voltages, switching transients
can exceed device voltage ratings.
At very low input voltages, supply voltage
to control and transistor drive circuits is
inadequate.
In all cases, the power supply should shut
down until nominal input voltages return.
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 6
Power supplies usually incorporate circuitry that inhibits the operation of the
power supply if the input supply voltage is too low or too high.
Low input voltages are stressful to a power supply because the input current
rises as the input voltages drop (assuming a constant load). Most AC power
supplies have a lower input voltage limit of 85 VAC to prevent overheating
the internal components. Excessively low input voltages can cause faulty
operation of the control circuitry, and insufficient gate drive voltage to the
MOSFETS and IGBTs.
High input voltages can damage transistors and other components as their
voltage ratings are exceeded. Be wary of switching generated transients
which at high input voltage conditions may exceed the transistor
specifications.
When the input voltage to a power supply is out of specification, the safest
response is to turn off the power supply until input conditions improve.
Page 6
Over Voltage Protection
“Crow-Bar”
Fail-Safe circuit to protect load if power
supply outputs excessive voltage
Power Supply
Fuse
Power
Converter
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Over
Voltage
Sensor
Load
SCR
Advanced Power Supply Topics
Slide 7
Over voltage circuits provide protection for the load in the advent of a failure
in the power supply.
A “Crow-Bar” circuit typically includes a Silicon Controlled Rectifier (SCR), a
fuse, and a voltage sensing circuit.
If an over voltage is detected, the SCR is turned on to a conducting state
and current from the power supply flows through the fuse and SCR into the
power return line (ground).
The fuse is sized to blow at the maximum current rating for the power
supply, but not at the normal current load experienced by a functional
system.
Fuse tolerances are quite wide, so the power supply must be rated for more
current than required by the load to insure that the fuse will blow when the
“Crow-Bar” is activated.
The cost of providing a crow-bar circuit, and associated cost of a power
supply with a higher current rating, must be weighed against the cost of
destroying the “LOAD” if a power supply failure occurs.
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Over Current Fault Response
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Immediate Shut down
Delayed shutdown
Constant Current Limiting
Constant Power Limiting
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 8
There are four basic behaviors a power supply can provide in response to an
over current situation:
1. The power supply can immediately shut down and then periodically check
to see if power can be restored. (Auto-Restart).
2. The power supply can tolerate the overload for a period of time before it
shuts down (perhaps a few minutes, it is thermally limited).
3. The power supply can enter a current limited mode. The full rated current
is delivered to the load. The output voltage is determined by the load current
and the impedance of the load. An issue with this response methodology is
that the power supply is providing high power to a load that may have
experienced a failure. This could become a fire hazard.
4. The power supply enters a power limited mode. The output voltage is
reduced (but controlled) as the output current grows. The goal is to limit
thermal overload. This mode is useful where the load can tolerate a voltage
drop.
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Voltage Trimming
Power
Supply
User output
voltage
adjustment
© 2006 Microchip Technology Incorporated. All Rights Reserved.
LOAD
Power Wiring
Resistance
Advanced Power Supply Topics
Slide 9
Voltage trimming is often supplied on a power supply to enable the user to
adjust the output voltage slightly to compensate for system wiring voltage
drops or to compensate for unit to unit variations of the power supply.
The voltage trimming is usually implemented with a potentiometer (variable
resistor) that the user adjusts with a screwdriver.
Page 9
Remote Sensing
Power
Supply
LOAD
Power Wiring
Resistance
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Remote Sense
connections
Advanced Power Supply Topics
Slide 10
Remote sensing uses additional wires from the power supply to the load to
sense the voltage at the load. These sense wires are called “Kelvin”
connections. No appreciable current flows through them so they do not
experience any significant voltages drops.
Remote sensing compensates for voltage drops across the system wiring
between the power supply and the load.
The remotely sensed voltage modifies the power supply output voltage over
a limited range (typically +-3%). The voltage adjustment range is usually
limited to insure that an open remote sense circuit does not produce
excessive output voltages that might damage the load.
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Power Supply Sequencing
sequential
simultaneous
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5.0v
3.3v
2.5v
V
5.0v
3.3v
2.5v
T
T
offset
ratio metric
V
5.0v
V
3.3v
2.5v
5.0v
3.3v
2.5v
T
T
Choose the method that meets system requirements
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 11
Many electronic devices require their multiple supply voltages be
coordinated at power on and off to protect their circuitry.
If the suppy voltages are improper applied, many integrated circuits can
experience “Latch-up”, a destructive process where internal
semiconductor junctions become forward biased resulting in uncontrolled
current flow.
Other systems require that specific circuitry be powered prior (to put it in a
known safe state) to the rest of the load circuitry.
The most common power supply sequencing method is the simultaneous
ramp up and down.
The choice of power sequencing is very dependent on the system
requirements.
Page 11
Load Sharing Issues
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Multiple power modules share load
Component and wiring differences cause
some modules to work harder than
others.
The heavily loaded modules get hotter
and reliability drops causing failures –
Domino Effect
Power
Module
Load Equalization
Comm. Channel
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Power
Module
Load
Power
Module
Advanced Power Supply Topics
Slide 12
Load sharing connects the outputs of two or more power supplies together to
power a load. Usually there are more modules with more total power
capability than required by a system. The amount of “redundancy” is
dictated by system reliability goals.
If a power supply module fails, the remaining modules continue supplying
power to the system. The user can remove failed modules and install a
replacements without powering down the system.
Load sharing is often a requirement for large computer system power
supplies where fault tolerance and high system reliability is a necessity.
Ideally, all of the modules should share the load equally. By operating at
reduced workload (assuming redundant capacity), the reliability of the power
modules will be increased.
Slight differences in component values and wiring resistances causes the
output voltage of some modules to be slightly higher than others. Even if the
voltage imbalance is only millivolts, the result is some modules “hog” the
load while others are lightly loaded.
The heavily loaded modules run hotter and will probably fail earlier, the
remainder carry more load and then they fail, geating a “Domino” effect.
A slow analog or digital communications link passes information among the
modules to force the output voltages to a common value to enforce load
sharing. The control loop is slow, working on a thermal time constant basis.
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EMI Filtering
80
FCC class A
70
dB
uv
VDE 0871
class A,C
60
VDE 0871
class B
50
FCC class B
40
30
10 KHz
50 100 150
kHz kHz kHz
500 kHz
1 MHz
10 MHz
30 MHz
Conducted mode RFI limits
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 13
Around the world there are many government agencies and standards
bodies that regulate conducted and radiated emissions of electronic
equipment. Europe, with the rise of the EU is migrating to a unified IEC
standards. Consult with experts on what the applicable limits are for your
markets.
The Switch Mode Power Supply with its high frequency switching and high
power levels is by nature a powerful emitter of radio frequency noise. The
chart shown in this slide is illustrative of the typical limits imposed on power
supply designers and manufacturers.
When the tests are performed, the RFI receiver has a fixed bandwidth,
typically 9 KHz wide when scanning from 150 KHz to 30 MHz, and 220 Hz
when scanning below 150 KHz.
The limited measurement bandwidth is important in understanding the value
of dithering (spreading) the noise emissions over a wide frequency range to
reduce the average noise power per Hertz of bandwidth. While the total
emitted noise energy does not change, the measured peak values will be
reduced.
Page 13
EMI Filtering Elements
Differential
mode
chokes
Xcap
Differential
mode
Ycap
common
mode
Vac
Common
mode choke
Gnd
Ground Leakage Currents
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 14
EMI filters for switch mode power supplies typically include circuit elements
to attenuate both common mode and differential noise.
The most common low cost filters incorporate the ”Xcap” (for differential
mode noise supression), the common mode choke, and the “Ycap (for
common mode noise suppression).
For applications that require more attenuation of EMI, complex filter designs
incorporate multiple instantiations of common mode and differential filter
elements.
The “Ycap” enables AC current flow into the ground connection. This
current is called “Ground Leakage”.
The “Ycap” ground leakage is an issue in medical applications where ground
leakages must be kept very small.
Page 14
EMI Filtering
Leakage Currents
Filter
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Safety agencies limit
leakage currents to 0.5 – 5
ma for general applications.
Medical equipment (patent
connected) limits < 100 uA.
Ycap
Vac
Gnd
Ground Leakage
Currents
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 15
Safety agencies have standards (such as IEC-950, and UL-1950) that limit
ground leakage currents to levels that are considered safe for the general
public. The major contributor to ground leakage currents are the common
mode filter capacitors called “Ycaps”.
The need to limit ground leakage current limits the minimum impedance of
the “Y” capacitors and therefore their effectiveness in combating common
mode noise.
The medical equipment has much lower limits (IEC-601, and UL-2601)
because equipment may be connected directly to patient internal organs
thus bypassing the skin resistance. Leakage currents can cause heart
defibrillation.
Meeting medical leakage current requirements can be difficult because the
parasitic capacitances between circuit components and the enclosures can
add significant leakage currents.
Page 15
Key Support Documents
Device Selection Reference
Document #
General Purpose and Sensor Family Data Sheet
DS70083
Motor Control and Power Conv. Data Sheet
DS70082
dsPIC30F Family Overview
DS70043
Base Design Reference
Document #
dsPIC30F Family Reference Manual
DS70046
dsPIC30F Programmer’s Reference Manual
DS70030
MPLAB®
DS51284
C30 C Compiler User User’s Guide
MPLAB ASM30, LINK30 & Utilities User’s Guide
DS51317
dsPIC®
DS51456
Language Tools Libraries
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 16
For more information, here are references to some important documents that
contain a lot of information about the dsPIC30F family of devices.
The Family Reference Manual contains detailed information about the
architecture and peripherals, whereas the Programmer’s Reference Manual
contains a thorough description of the instruction set.
Page 16
Key Support Documents
Microchip Web Sites: www.microchip.com/smps
www.microchip.com/16-bit
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Slide 17
For device data sheets, Family Reference Manuals, and other related
documents please visit the following Microchip websites.
Page 17
Thank You
Note: The Microchip name and logo, dsPIC, MPLAB and PIC are registered trademarks of Microchip Technology
Inc. in the U.S.A. and other countries. dsPICDEM, dsPICDEM.net, dsPICworks, MPASM, MPLIB, MPLINK and
PICtail are trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All other trademarks
mentioned herein are property of their respective companies.
© 2006 Microchip Technology Incorporated. All Rights Reserved.
Advanced Power Supply Topics
Thank you for attending this seminar.
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Slide 18